Cutting chromosomes gives clues to cell division

When we need new cells in our body, for example, to replace dead or damaged cells, they don't just appear from nowhere – they are created by the division of one cell into two new daughters. This process is called mitosis. Now scientists at the University of Michigan have used a clever laser technique to get an even closer insight into how mitosis works, and how it might go wrong in diseases like cancer, where cells divide out of control.

Normally in mitosis, cells copy all their DNA, then line up these two sets of chromosomes in the middle of the cell, thanks to a microscopic scaffold-like structure called the spindle. The spindle grows from each side of the cell, and attaches to the chromosomes, eventually pulling them apart in opposite directions. If this goes wrong, then the new daughter cells may end up with the wrong number of chromosomes, which is bad news for the cell.

For many years, researchers have tried to understand how the cell manages to divide its chromosomes equally between daughters, and now new research in this week's edition of Current Biology by Alan Hunt and his team may help to explain why. The Michigan team used high speed lasers to slice off tiny pieces of chromosomes from within living, dividing newt cells, and watched what happened. The pulses of laser light lasted for only a femtosecond -a billionth of a millionth of a second, but they were enough to cut the chromosomes.

Previously, researchers have though that something called 'polar ejection forces' are at working in dividing cells, helping to maintain the pull across the spindle, and ensure that an equal number of chromosomes go to each new cell. Hunt suspected that these forces should be directly related to the size of the chromosomes, so cutting off measurable pieces should have proportionally measurable effects on cell division – and that's what they found. They also discovered that these polar ejection forces are an important physical cue that helps to directly control mitosis, and the direction of chromosome movements.

Not only does this discovery shed light on cancer – and on chemotherapy, which often works by blocking mitosis in cancer cells – but it could also help us to understand genetic diseases such as Down's Syndrome, which are caused by an incorrect number of chromosomes during egg formation.

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